Ion exchange is generally defined as a reversible chemical reaction in which ions are exchanged between a solution and an insoluble solid. More specifically, it is a type of filtration in which an ionized compound or element changes place with another ionized compound or element on the surface of a medium. The term “ion-exchange capacity” describes the total available exchange capacity of an ion-exchange medium, as described by the number of functional groups on it.
The process of ion exchange is useful over a broad range of applications, and may generally be categorized as either anion or cation exchange. Ion exchange is most frequently used to achieve high-purity water (including softening, deionizing, water recycling, and heavy metals removal and recovery from wastewater) and in chemical-related processing. Ion-exchange media are also useful in chromatography, catalysis, electrochemical processes, the creation of super acids and super bases, and for the separation, concentration and/or purification of ionic species, pharmaceutical separations technology, the treatment of radioactive waste, sugar refining, etc. These materials take a variety of forms, including naturally occurring ion exchangers, synthetic ion exchangers, composite ion exchangers, and ion-exchange membranes.
Most typically, ion-exchange resins are used. The most common form of an ion-exchange resin is a synthetic insoluble matrix of styrene and divinylbenzene copolymers cross-linked to form beads between 0.03-1.0 mm. The beads must be activated to function as ion-exchange material. The beads can be converted to cation-exchange resins through sulfonation or anion-exchange resins through chloromethylation. Ion-exchange resins are capable of removing heavy metals, such as lead and mercury, from solution and replacing them with less harmful elements such as potassium or sodium. The process for the production of these resins is expensive. The product resin beads are also susceptible to fouling due to organic contaminants in the water flow. This necessitates the use of activated carbon or other removal technologies prior to ion-exchange treatment, only further complicating the process and adding to the cost.
Developmental approaches to the production of an activated carbon media involve using natural organic materials as a source. Examples of such organic materials include a variety of vegetable materials, softwoods, cornstalks, bagasse, nut hulls and shells, various animal products, lignite, bituminous coal, straw, anthracite and peat. These processes have largely focused on either chemical activation (e.g., sulfonation or chloromethylation) or full physical activation of the starting material at high temperatures. It is known in the art to convert sources such as sawdust, wood, or peat into an adsorber by chemical activation. For example, peat is impregnated with a strong dehydrating agent, such as phosphoric acid or zinc chloride, mixed into paste and then heated to a temperature of 500-800° C. to activate the peat. The product is then washed, dried and ground to a powder. In such a process, the resultant product generally exhibits a very open, porous structure that is ideal for adsorption of large molecules. Additionally, a process of steam activation, also known as physical activation, is typically employed with sources such as coconut shell and bamboo. The starting material is often activated by exposure to steam or carbon dioxide at high temperatures. Temperatures that have been used in the art include about 650-1200° C. These processes do not produce a media with a usable ion-exchange capacity.
One of the most significant challenges in producing an ion-exchange medium from natural, organic constituents is achieving a balance between the physical integrity of the form of the ion-exchange medium and the ability of the medium to serve as an ion-exchanger. The source of the starting material and the method of producing a medium from the precursor are the two most important variables determining the usefulness of the final product as an ion-exchange medium. Importantly, the process used to activate or partially activate the organic material will also determine the hardness of the resultant granule and its ability to function as an ion-exchange medium.
One significant disadvantage of the prior art is related to the resultant medium's capacity to function as an ion-exchange medium. Partially decomposed organic starting material inherently possesses ion-exchange characteristics; however, the material often loses its ion-exchange functionality during pyrolysis. Pyrolysis is simply the chemical decomposition of a substance by the exposure of extreme heat. Most natural organic ion exchangers tend to have weak physical structures making their application possibilities limited. Because the organic material is prone to crushing, it does not stand up to the often rigorous processes used in ion-exchange applications. Additionally, many known processes include the step of carbonization either prior to or concurrent with activation. Carbonization may cause considerable shrinkage and weight loss of the feedstock. Organic sources also generally have non-uniform physical properties. Naturally occurring organic ion exchangers are unstable outside a moderately neutral pH range. Finally, such organic ion exchangers tend to be prone to excessive swelling and peptizing.
Natural inorganic ion exchangers also have a number of disadvantages. They, too, tend to have relatively low ion-exchange capacities. Like natural organic ion exchangers, natural inorganic ion exchangers tend to have low mechanical durability. Because they are prone to degradation when exposed to certain chemicals in solution, such as oxidizing agents, it may be necessary to pretreat natural inorganic ion exchangers.
The use of synthetic organic ion-exchange resins similarly has disadvantages. Importantly, resins generally have the disadvantage of foulant formation on the resin beads. Ion-exchange material removes some soluble organic acids and bases while other non-ionic organics, oils, greases, and suspended solids remain on the surface of the resin. This process is known as fouling. Foulants can form rapidly and can significantly hinder performance of the system. Cationic polymers and other high molecular weight cationic organics are particularly troublesome at any concentration. For certain types of resins, even 1 ppm suspended solids can cause significant fouling over time. As such, prefiltration upstream of the ion exchange might be required to remove elements, such as colloidal silica, iron, copper, and manganese that can cause fouling of the resin. As organic contaminants begin to build up on the surface of a resin, the flow of other particles and bacteria is also diminished. The costs of pretreatment can be significant.
Additionally, resins require regeneration once the ion-exchange sites have been exhausted, for example, as feedwater flows through a bed. During regeneration of a cation resin, cations that were previously removed are replaced with hydrogen ions. A step known as “backwash” is often employed during regeneration so that any organic contaminant build-up in the resin can be relieved allowing free flow through the resin. Chemically regenerated ion-exchange processes known in the art use excessive amounts of regeneration chemicals, require periodic and sometimes even ongoing treatment, and disposal of the chemical waste. The processes can be complex and expensive to operate. There is still a need for a process with decreased chemical requirements in the production of ion-exchange media.
While the processes known in the art for the preparation of ion-exchange material from natural solid organic material have been useful for certain ion-exchange applications, for particular applications it is necessary to increase the hardness of the resultant ion-exchange medium while minimally sacrificing the media's cation-exchange capacity in the process. It is necessary to develop a process for the low-cost production of an ion-exchange medium that has good ion-exchange capacity, organics adsorption capabilities, and improved strength such that the medium may be used in a wider range of applications.
The present invention is an improved process for the production of an ion-exchange medium which possesses increased physical integrity of the medium without compromising the natural cation-exchange capacity of the starting material.
This invention is related to a natural organic starting material, and in particular to the use of decomposed or partially decomposed organic matter. More specifically, a preferred starting material is peat or leaf compost material. Unlike other types of organic materials found in nature, peat is naturally partially carbonized. Because of this inherent characteristic, peat naturally possesses a cation-exchange capacity of approximately 120 meq/100 g. It has been discovered that much of the naturally occurring high cation-exchange capacity may be retained if the peat is subjected to either steam, carbon dioxide, nitrogen or other inert media at low activation temperatures in an inert environment.
In general, enhanced mechanical strength and dimensional stability have been achieved when decomposed or partially decomposed organic matter has been partially physically activated at low temperatures. The resultant medium will also possess enhanced organic contaminant retention capabilities when in wastewater. The present inventive process additionally has been found to decrease the amount of leaching into treated water from tannins that are naturally present in certain starting materials. These improvements in the process permit the resultant partially activated media to be used in a broader range of applications than those seen in the prior art.
As used herein, the following terms have the meanings given below, unless the context requires otherwise.
The term “mEq” means milliequivalents. The equivalent is a common unit of measurement used in chemistry and the biological sciences. It is a measure of a substance's ability to combine with other substances. The “equivalent” is defined as the mass in grams of a substance which will react with 6.022×1023 electrons. Another way of defining an “equivalent” is the number of grams of a substance that will react with a gram of free hydrogen. The equivalent weight of a given substance is approximately equal to the amount of substance in moles, divided by the valence of the substance. Because, in practice, the equivalent weight is often very large, it is frequently described in terms of milliequivalents (mEq). A mEq is 1/1000 of an equivalent.
The term “hardness” means a property of the granular medium's ability to resist attrition during handling and operation. The “hardness number” is a measure of this property and is determined by way of the “Ball-Pan Hardness” test. The higher the value, the less the losses in uses. A certain amount of material is put into a pan, together with some steel balls, and shaken for a defined period of time. The material is weighed before and after the shaking to determine the amount of attrition. The percent of original material that remains after shaking is the “hardness number.”
The term “iodine number” means an equivalent to the surface area of activated carbon in mg/g. It is the most standard fundamental parameter used to characterize activated carbon materials performance.
The term “empty bed contact time” means the time required for a liquid in a carbon adsorption bed to pass through a carbon column, assuming all liquid passes through at the same velocity. It is equal to the volume of the empty bed divided by the flow rate.
The term “about” means approximately or nearly and in the context of a numerical value or range set forth herein means±2% of the numerical value or range recited or claimed.
The term “ug” means one microgram or one one-millionth of a gram or one one-thousandth of a milligram.
The term “ng” means nanograms or 1×10−9 grams or 0.000000001 grams.